The first thing I'll address is midi-chlorians, mostly just to get it out the way. According to Wikipedia midi-chlorians are "...intelligent microscopic life forms..." that "...could allow their symbiont to detect the pervasive energy field known as The Force." If I'm understanding this explanation and the one given in The Phantom Menace correctly it basically means that if someone has a lot of these little bacteria (or whatever) then they can use the force. The more you have, the more powerful your potential. There's also a fan-theory kicking around that rather than midi-chlorians giving someone a connection to The Force, it is the midi-chlorians that are attracted to people who are already Force-sensitive. Personally, if we absolutely must have midi-chlorians in our Star Wars, I prefer the fan theory.

Without midi-chlorians I like The Force to be mostly a vague mystery. Something along the lines of 'The power of the universe' is good enough for me. Not unlike achieving a state of zen or nirvana in our world, becoming in-tune with The Force comes down to a person's state of mind, their own discipline and, hokey as it may sound, being 'one' with the universe. I'm not sure it ever needs to be anymore complicated than that.

In regards to the 'light' and 'dark' 'sides' of The Force I've always figured these were just labels given by the Jedi and Sith. In the Extended Universe, if I recall correctly, there are many other 'religions' that follow The Force, their perspectives of it different, but no less right or wrong than the Jedi. In short, it's not as black and white or definitive as the Jedi believe, that's simply their belief doctrine. Other Force-users that aren't Jedi or Sith follow different sets of rules where there is no 'light' or 'dark', some viewing the The Force much more simply and others perhaps far more complex.